The present invention is particularly useful in a printer apparatus wherein a web of dye donor is advanced from a supply roll, past a thermal print head, to a motorized take-up roll. Referring to FIG. 1, a thermal printer 10 includes a print head assembly 12; dye donor web supply roll 14 and take-up roll 16, contained in a cartridge 17; a roller platen assembly 18; a pair of pinch rollers 20 and 22; a dye receiver medium transport guide 30; and a dye receiver medium supply 24.
Normal thermal printer operation includes loading dye receiver medium, printing information upon the dye receiver medium, and ejecting the finished print. Each of these operations is fully described in commonly-assigned U.S. Pat. No. 5,176,458, which issued on Jan. 5, 1993. Therefore only a brief description will be given of the illustrated embodiment of the thermal printer.
Printer operation begins with a loading phase, in which a sheet 28 of dye receiver medium advances from supply 24 along guide 30 to a gap between print head assembly 12 and platen assembly 18. The leading edge of sheet 28 is held in the nip of rollers 20 and 22. Print head assembly 12 moves toward roller platen assembly 18, pressing dye donor web 26 and the dye receiver medium 28 against roller platen assembly 18, to form a sandwich for thermal printing.
Print head assembly is comprised of a print head 13 and a heatsink 15. Referring to FIG. 2, the print head 13 of print head assembly 12 includes a plurality of heating elements 32, such as electrical resistors. When one of a plurality of switches 34 is closed, the associated heating element 32 is connected to a voltage potential source V.sub.S.
Dye donor web 26, shown in FIG. 3, comprises a leader portion 27 followed by a repeating series of dye frames. The dye frames may be contiguous or spaced by interframe regions 29, as shown in FIG. 3. Each series includes in sequence yellow, magenta, and cyan dye frames. A single series is used to print one color plane on dye receiver medium 28. Dye donor web 26 may also be black for printing text, and for other applications.
Two LEDs 36 and 38 illuminate the dye donor web from above. LED 36 emits yellow light and LED 38 emits red light. Two photodetectors "A" and "B" are disposed below the dye donor web and receive light which passes through the dye donor web. Photodetectors "A" and "B" provide signals for identifying the start of each series, and each individual color dye frame in such series. For a more complete discussion of this identification system, reference is made to commonly assigned Reissue U.S. Pat. No. Re. 33,260 to S. Stephenson.
A problem with thermal head printers, is that the print head must be cooled. In particular, the print head must be cooled in a uniform manner to achieve uniformity of image intensity. Prior art devices incorporated cooling fins, ducts and fans, see for example U.S. Pat. No. 5,374,944, assigned to the same assignee as the present invention. Some of these prior art devices are either, uneconomical because they require a separate fan to blow air over a heatsink attached to the thermal head, or inefficient because air circulated through the entire machine is also used for cooling the print head. When using air circulated through entire machine to cool the print head, the air must be circulated at a higher volume than would be necessary if the air were blowing directly on the print head heatsink.